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Measuring waves

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Quiz

True or false? The unit of amplitude is hertz.

Frequency

How often something occurs. More specifically, the number of waves that pass by a point in space every second.

Amplitude

The maximum distance moved by a point, on a vibrating body or sound wave, from its rest position.

Hertz

A unit for measuring the frequency of energy waves, like sound or electricity, in oscillations per second.

Wavelength

The distance from one wave of energy to another as it travels from one point to another. In a sound wave it is the distance between two peaks, or two troughs.

Waves have many different properties, many of which can be observed and measured with a simple experiment. You need a long, narrow, plastic tank, water, a ruler, a pen, a sheet of paper, a stopwatch, and someone to do the experiment with. First, fill the tank half-way up with water. Then, try to make some waves, by gently rocking one side of the tank back and forth. You can see that the surface of the water is moving — you’ve just created waves!

Keep a steady pace when rocking the tank back and forth, to make sure that the waves don’t change much. While you are moving the tank, the other person can measure how high the waves are. To do that, they should simply stick the ruler into the tank, and note the height of the wave peak. Do this a few times, and calculate the average. The height of the wave you just measured is its first property — its amplitude!

Next, measure the distance between the two closest peaks of waves, by placing the ruler on the side of the container. Do this a couple of times, and then calculate the average. You’ve just measured another property of the wave! This distance between the two closest peaks is the wavelength of the wave. Wavelength is usually measured in metres, or in this case centimetres — as the tank is quite small.

Now, mark a point on the edge of the tank. Set the time to 10 seconds, and count how many wave peaks pass that point in those 10 seconds. Again, do this a couple of times, and then calculate the average. To find out how many peaks cross the point every second, divide that average number by 10. You’ve just measured and calculated another property of the wave!

The number of peaks that pass through a given point every second, is the wave’s frequency. The unit of frequency is hertz. Using the wavelength and frequency you just measured, you can calculate another property of waves. If you multiply wavelength and frequency, you get the speed of the wave!